1. Field of the Invention
The present invention generally relates to a projection screen of a reflection type for displaying a picture by means of a linearly polarized light and, more particularly, to the projection screen suited for use with a liquid crystal display device.
2. Description of the Prior Art
As compared with the display device utilizing a cathode ray tube, the liquid crystal display device has many advantages: compact in size, light-weight and less harmful to the environments when disposed of. Therefore, the liquid crystal display is now in widespread use. However, the liquid crystal display device has a problem in providing a sufficient brightness when imagewise ray of light descriptive of a series of pictures reproduced by the liquid crystal display device are projected onto the large-size screen. Therefore, viewers are generally required to look at the screen in a dark room in which entry of external light from the outside is minimized.
In view of the finding that, while rays of light projected from the liquid crystal display device are linearly polarized, external rays of light that constitute a cause of reduction in brightness of the image projected on the screen are generally non-polarized, the image projected onto the screen under bright environments can be reasonably viewed if the screen is so designed as to eliminate a reflection of the external rays of light impinging thereon from the screen. By way of example, Japanese Laid-open Patent Publications No. 62-266980 and No. 64-77085, published in 1987 and 1989, respectively, disclose the use of a light absorbing polarizing filter on one surface of the screen to minimize the amount of reflection of the non-polarized external rays of light from the screen surface.
Also, Japanese Laid-open Patent Publication No. 2-267536 published in 1990 discloses an image projection system wherein a source of light such as a house room illuminator is covered up with the use of a light-absorbable polarizing filter having a direction of polarization oriented perpendicular to the direction of polarization of the polarizing filter on the screen.
It has, however, been that, in order to enhance the contrast or visibility of the linearly polarized image projected onto the screen under bright environments, a mere elimination or minimization of the non-polarized external rays of light is not sufficient, and the following problems which will now be discussed in detail with reference to FIG. 15 have to be eliminated.
As shown in FIG. 15, the prior art light-absorbable polarizing filter comprises a polarizing sheet 6 sandwiched between protective films of triacetyl cellulose (TAC) generally identified by 5. The polarizing sheet 6 used therein is in the form of a stretched film of poly(vinyl alcohol) (PVA) in which polarization developing substances are dispersed. This prior art light-absorbable polarizing filter has the following problems.
(a) Due to process limitations generally encountered in the process of stretching the PVA film, the polarizing filter that can be manufactured has a width generally limited to 50 to 100 cm and, where this polarizing filter is desired to be used for a screen of a size larger than the polarizing filter, a number of the polarizing filters must be joined together to provide a surface area corresponding to the desired screen size. The fact that the plural polarizing filters are joined together involves the presence of joints at which the polarizing filters are joined, and these joints tend to constitute a cause of reduction in quality of the image eventually projected on the resultant screen.
(b) The PVA film used in the prior art polarizing filter has a film thickness of about 20 .mu.m and is therefore very pliable. On the other hand, the TAC film that is applied on each of the opposite surfaces of the PVA film for the protection of the polarizing sheet has a film thickness of about 80 .mu.m and is therefore relatively rigid. Accordingly, the screen comprising the polarizing sheet sandwiched between the protective films lacks a flexibility generally required in the screen, involving a difficulty in bending and capability of being embossed.
(c) Since the polarizing sheet is flat in shape, disposition of the polarizing sheet in the screen tends to result in an increased local reflection of an image of the aperture of the liquid crystal display device to such an extent as to reduce the quality of the image projected on the screen considerably.
(d) While iodine is generally used as a polarization developing substance to be employed in the polarizing filter capable of exhibiting a high polarizability (99.9% or higher polarizability), the polarizing sheet using iodine has a low resistance to heat and also to humidity and is, therefore, susceptible to reduction in polarizability and transmissivity of polarized light.
It is generally well known that the liquid crystal display device is unable to make a maximized utilization of the light produced by a light source partly because a video signal applied to the liquid crystal display device is reproduced with the use of a polarizing filter of light absorbing type and partly because the aperture of the optical system thereof is small due to thin-film transistors disturbing a transmission of signal light therethrough. Accordingly, the screen used to project the imagewise rays of light emitted through the liquid crystal display device is required to have as high a reflectivity as possible and, therefore, this requirement brings about the following problem.
(e) If oxygen pass through material covering a surface of a reflector sheet, a metallic layer forming a part of the reflector tends to be oxidized in contact with air, resulting in an eventual reduction in reflectivity exhibited by the reflector sheet.
Particularly in the case of a large-size screen, the screen is generally folded or rolled up for storage when it is not in use. Therefore, this brings about the following problem.
(f) A finely indented structure employed in the reflector sheet for optimization of the angle of reflection of rays of light projected onto the reflector sheet is susceptible to damages when the screen is repeatedly folded or rolled up each time the screen is to be stored.